1. Field of Invention
The present invention relates to processes for burning and/or combustion of solid and/or liquid fuels and more particularly to the treatment of the solid residues generated by such processes in the form of cinders and fly ash.
2. State of the Art
When, for example, household garbage is burnt, approximately 250 kg of cinders and 25 kg of fly ash, which is entrained in the exhaust gases, are generated per metric ton. In other combustion processes the quantity of residues depends on the fuel employed and the proportion of cinders/fly ash depends on the type of combustion. Experience shows that the cinders, although quasi-vitrified, are more or less coated with soluble compounds, for example chlorides and alkaline materials which are soluble in the case of the burning of household garbage. Quenching of the cinders in a cinder pit is intended merely to cool these cinders but not to wash them; in most cases this amounts to adding water to the cinder pit, to compensate for the evaporation and the residual moisture of the cinders. In very rare cases it has been possible to employ a high water flow rate to wash the cinders; the latter have then exhibited good qualities for reclaiming. Furthermore, in household garbage incineration plants it is known to use known processes for removing metal scrap from the cinders and to crush/screen them to give them a particle size making them potentially suitable for reclaiming as highway foundation material or as material for construction. For this reclaiming to be really possible there must be no significant release of soluble components in the presence of water. Good performance is therefore necessary in a leaching test. Utilization of the known washing processes would make this possible, but this would involve a high water consumption.
Fly ash very often also includes a high proportion--which may reach 30%--of easily soluble compounds, including heavy metals. It must therefore be treated before it can be stored definitively in a dump or be reclaimed. Various processes have been proposed for such a treatment. For example, in the case of incineration plants, an electrostatic precipitator is frequently found downstream of the incineration furnace and the recovery boiler, ensuring the trapping of the fly ash followed by a wet purification ensuring the scavenging of the acidic gases, HCl and HF in particular. It has been proposed to employ the acidic purge from such wet purification for treating the fly ash by leaching in an acidic phase. The disadvantage of such a method--besides the introduction of a complex external plant--is the use of a liquid that is rich in solubilized heavy metals and in trace elements scavenged in the wet purification. It is then impossible to obtain good leaching properties of the treated residue, in particular with regard to the total soluble fraction, without high water consumption.
It has also been proposed to introduce this fly ash into the recycled scrubber liquid ensuring the scavenging of the acidic gases. In this method there is no need to introduce a complex external plant but, bearing in mind the characteristics of the scrubbing liquid, it is also impossible to obtain good leaching properties of the treated residue, in particular with regard to the total soluble fraction, without high water consumption.